There are a number of applications for trace element analysis. Typically, low parts-per-million analysis requires a traditional laboratory environment, and thus cannot be conducted in the field. The laboratory equipment used is relatively large and expensive, and is run by trained technicians.
One application for trace element analysis is gemology, and in particular the detection of characteristics of stones. The Determination of country of origin and treatments and/or enhancements of gemstones has had a profound affect on the value of gem materials. The origin of a stone may be determined based on several significant factors that are fundamentally determined by the geo-chemical environment in which the minerals and/or gems are created. Variations in chemistry, temperature and pressure all affect the structure of the final crystals produced, which serve as a “signature” for a particular geographic area. Changes in these critical components will yield changes in elemental chemistry, physical and/or optical properties and internal characteristics, e.g., secondary micro minerals, which are entrapped in the host crystal during the growth process. Isolating patterns and small changes in optical, physical, elemental characteristics and inclusions commonly provide diagnostic tools for country of origin and enhancement determinations.
Historically, the application of heat to various materials alters the appearance of the gemstones, which when judiciously applied, improve their desirability and salability. These changes can have a significant affect on the value of a gemstone. In the early to mid 1960s, more sophisticated heat treatment procedures were developed to achieve more radical alterations in appearance and in subsequent value. High temperature heat treatment has had a significant impact on the gem market, as well as, the detection of country of origin. Recently, light elements have been added to the heating process along with various fluxes, to produce even more drastic changes in color and appearance. This process has further complicated the detection of origin and the separation of natural from artificially altered or colored gem material.
Because of the nature of the gem market, stones which have been treated are generally worth less than corresponding untreated stones which have a similar appearance, due to the relative rarity of the untreated stone. There is therefore an incentive to try to pass off treated stones as untreated stones, to develop new and more difficult to detect treatments, and methods of determining whether a stone has been treated. It is also useful to determine an origin of a stone, since this may also influence its value, and help distinguish whether a treatment has taken place to alter a characteristic of the stone.
Detection of these three critical value-related gem treatments/enhancements have been via microscopic, spectral and elemental techniques, that examine physical, optical and chemical characteristics of the stone. However, to date, each method has demonstrated significant shortfalls in one or more of reliability, efficiency and cost per analysis.
Microscopic techniques require a high level individual of experience and expertise, as well as, requiring a substantial and reliable sample set and a thorough familiarity of characteristics associated with particular origin. In addition, the observer must be familiar with the impact of high temperature alteration and various properties and internal characteristics. Microscopic techniques are typically non-destructive.
Spectral characteristics can also be useful diagnostic tools to determine both origin and heat alteration. Spectral techniques are also typically non-destructive.
The most complex of these three techniques is the elemental analysis. Except for spectral techniques, which have insufficient sensitivity for distinguishing the various origins and treatments, elemental techniques are quasi-non-destructive. That is, while removal of a physical sample of the specimen is required, the sample is small, and the remaining portion of the stone is typically available for its intended purpose without substantial impairment of value, and indeed, as a result of the test, may have an increase in value. The particular challenge addressed by this technique is the detection of the addition of light elements, which have the ability to radically alter appearance, and potentially the value of the stone. These light elements include, for example, Beryllium (Be), Lithium (Li), Magnesium (Mg) and Potassium (K), which are added to the flux environments in which the gems are heated. Penetration of a treatment can vary in depth from near surface to throughout the entire stone. The flux itself is part of a treatment which serves to fill cracks and voids in the stone, potentially improving optical clarity.
To date, SIMS (Secondary Ion Mass Spec) has been used to determine elevated levels of Beryllium and related elements. SIMS is expensive, complicated and requires a high level of expertise to operate and interpret data.
ICP-MS (Inductively Coupled Plasma Mass Spectrometry) has also been successfully used to accomplish detection of these light elements. However, its high maintenance cost, coupled with complexity, limits the population of potential users.
LA-ICP/AES-OES (Laser Ablation Inductively Coupled Plasma Atomic Emission Spectrometry-Optical Emission Spectrometry) is a viable laboratory option, which typically employs a non-portable laser ablation system, and has a short sampling time, while maintenance and complexity of use are within the capacity with a reasonably skilled technician.
Laser Induced Breakdown Spectroscopy (LIBS) is known as a good candidate for in situ chemical analysis of geological specimens, since the technique can be performed on a small sample with little or no preparation. It provides a multielement analysis coupled with good sensitivity, very high spatial resolution, and depth profiling capability.
LIBS is also known as Laser Spark Spectroscopy (LASS), and Laser-Induced Plasma Spectroscopy (LIPS). LIBS is a form of atomic emission spectroscopy in which a pulsed laser is used as the excitation source. The output of a pulsed laser, such as a Q-switched Nd:YAG, is focused onto the surface of the material to be analyzed. For the duration of the laser pulse, which is typically 10 nanoseconds, a very high power density is present at the surface of the material. For example, the power density in excess of 1 Gigawatt/cm2 is possible using only a compact laser device and simple focusing lenses. A small amount of material from the surface of the material to be analyzed is vaporized from the surface by a process known as laser ablation, and the dielectric breakdown of the medium by the focused laser energy excitation near the point of ablation causes a short-lived but highly luminous plasma with instantaneous temperatures reaching 10,000° C., and supersonic expansion. A portion of the ablated material forms part of this plasma, which includes highly ionized atoms. After the laser pulse subsides, the expanding plasma cools. During this time the excited ions and atoms emit characteristic optical radiation, unmasked by the laser excitation and plasma emission themselves, as the atoms drop to lower energy states and recapture electrons. A time-gated spectral analysis of this radiation yields information on the elemental composition of the sampled material. By selecting gate timing, atoms having similar bandgaps, but differing characteristic recapture timeconstants, can be distinguished, and the effects of the excitation pulse itself minimized. Such gate delays may be from 1-50 microseconds.
Since the initial sample is small, and the laser ablates the material near the focal point, sequential samples allow a depth-sensitive analysis. Because the technique does not require a collection of the sample into a protected sample space, LIBS can be performed remotely. Likewise, the laser itself may be used to clean the sample surface prior to analysis.
The LIBS instrument may be calibrated to perform quantitative measurements of minor elements within a matrix material. Known examples include chromium in steel, magnesium in aluminum alloy, iron in glass, copper in copper sulphate solutions. Calibration may be achieved using matrix-matched certified reference materials containing various amounts of the analyte to be measured. Without controlling for the matrix composition, the technique can only be semi-quantitative, since the amount of ablation and fractionation effects are difficult to accurately predict. An internal standardization may also be performed, by quantifying the analyte of interest and the bulk matrix components, and determining a ratio; however, where the analyze may be 5-6 orders of magnitude below the bulk matrix, such methods may introduce substantial errors. On the other hand, such internal standardization may be used in conjunction with external standards to correct for potential variability and imprecision in the system itself. Internal standardization may also be applied to a plurality of analytes, yielding relative quantities within the bulk matrix.
In many cases, only one laser pulse is needed to analyze a material, allowing a large number of samples to be rapidly identified. On the other hand, the application of a double (or multiple) pulses on the sample surface or intersecting the plume at a different angle (e.g., 90° to the initial pulse) is also known, resulting in observed increase of the sensitivity of the technique. The improvement of the double pulse LIBS figures of merit are related to the enhancement of the emission line, increase of contrast and reduction of ignition threshold.
Other applications of trace element analysis include mineralogy, geology, oceanography, meteorology, environmental science, biology, forensic investigation, art materials analysis, and archeology. In each case, the availability of an instrument for field use which would provide quantitative trace element analysis at relatively low cost is lacking.